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<head>
    <meta charset="UTF-8">
    <meta name="viewport" content="width=device-width, initial-scale=1.0">
    <title>Document</title>
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    <script>
        // 定义变量接收
        var num = 1
        var sum = 0
        for(var i = 1;i<=20;i++){
            // 算阶乘
            num *= i
            // 算和
            sum += num
        }
        // 打印
        console.log(sum);
        
    </script>
</body>
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